The traditional view in transmission networks is to consider interference or collisions to be harmful. This is especially true with wireless networks where the spectral bandwidth available is limited. Wireless networks have thus been designed to prevent transmissions from interfering among each other.
Classical techniques to avoid interference include the reservation of the wireless channel using e.g. frequency division multiple access (FDMA), time division multiple access (TDMA), carrier sense multiple access (CSMA) or its variants. A more advanced approach to improving spectrum utilization has been by secondary usage of licensed spectrum where unlicensed devices listen on the channel and transmit only when found idle from licensed devices. Even in such an advanced “cognitive radio” approach, the idea is that unlicensed secondary devices avoid causing harmful interference to licensed devices.
In cases where interference does occur or cannot be avoided, multi-user detection (MUD) techniques have been used at the receiver side to disentangle interfered signals originating from separate transmitters. The MUD approach in general, however, requires more complex hardware at the receiver.
Recently, there has been a trend to exploit interference in order to improve the efficiency by which radio spectrum is utilized, thereby increasing the network capacity. For instance, the concepts of analog network coding (ANC) and physical layer network coding (PLNC) were introduced. Network capacity is improved using either ANC or PLNC by allowing interfering concurrent transmissions to occur.
With these techniques, certain nodes listen to transmissions and then forward the analog signals to destination nodes where interference cancellation is applied to retrieve the signal of interest. However, such techniques present several limitations
To illustrate these limitations, reference is now made to FIG. 1, which shows an example of a typical network comprising seven nodes A to G.
In FIG. 1, when nodes A and B desire to send respective data packets p1,p2 to each other, they usually do so directly and sequentially (as indicated with dotted lines), after that the nodes have captured the channel through CSMA for instance, or once transmission slots have been allocated through TDM in another example.
If the network uses existing ANC or PLNC schemes, nodes are allowed to transmit concurrently when an intermediate router node (in the present example, node C) exists between them. For that purpose, both nodes A and B transmit respective analog signals P1 and P2, which contain respectively data packets p1 and p2, to the intermediate router node C, which in turn returns the resulting combined analog signal P1+P2, comprising therefore both packets p1 and p2, to both nodes A and B. Each of these nodes A and B, receiving such a combined analog signal P1+P2 and knowing the signal it has sent itself (P1 for node A, P2 for node B), can deduct the analog signal it should receive (P2 for node A, P1 for node B), by removing the signal it has sent itself from the received combined signal. This scheme results in the use of fewer channels, thus leading to an increase in throughput.
However, in existing networks such as wireless local area networks (WLANs), there are no mechanisms that permit the use of ANC or PLNC schemes. The transmitting nodes have to synchronize their transmissions to ensure that packets collide so that decoding is later possible.
In particular, to realize the potential gains offered by ANC or PLNC schemes, two major issues arise. First, it is necessary to define how the nodes determine which transmissions are mixed in order to decode them. Secondly, it is necessary to determine the start and end times of transmissions that are allowed to be mixed.
To cope with the first above-mentioned issue for instance, a complex solution is proposed in the article “Embracing Wireless Interference: Analog Network Coding”, Katti et al., for the special case of MSK (Minimum frequency Shift Keying) modulated signals. Such a complex solution consists in measuring the variance of the received energy, and use the deviation from the constant signal energy property that holds for MSK in order to estimate the time instant which two packets collided. This method holds only for MSK signals. Further, additional hardware is required for the variance estimator. In addition, this process has to be executed throughout the reception phase of a packet since the receiver does not know the exact moment that collision will occur.
The mechanism used by the authors in Katti et al. to cope with the second issue consists in introducing known pilot bit sequences at the beginning and end of each packet (i.e. in preambles and postambles respectively). This is needed so that they identify the start and end of a packet that is needed for analog decoding. The disadvantage of this approach is that every frame at the physical layer must be extended with both a preamble and a postamble, even when no opportunities for using an ANC or PLNC scheme arise. Another disadvantage is that the preambles and postambles have to be received without interference, so that the receiving node can correctly identify the start of the packet. By randomizing transmissions as done in Katti et al., it is possible that this condition is not always satisfied.
It is thus an object of the present invention to overcome the above-identified difficulties and disadvantages by proposing a protocol which allows synchronisation of the transmission of colliding packets. Such a protocol overcomes several of the limitations of ANC or PLNC schemes that arise in practice in distributed wireless networks.